Monolithic catalysts consist of single blocks arranged with small parallel channels. In comparison with general catalyst, the monolithic catalysts show higher efficiency, lower power consumption, higher mechanical strength, and etc., and can be widely used in the fields of catalyst for automotive emission control, catalytic purification of toxic and hazardous air (such as volatile organic compounds, and nitrogen oxides), and petrochemical industry (such as hydrogen peroxide generated by anthraquinone process).
Generally, methods for preparing monolithic catalyst includes 3 types: (1) direct extrusion of prepared catalyst, disclosed in U.S. Pat. No. 4,912,077; (2) loading of active component on a monolithic carrier (such as active carbon, molecular sieve, and alumina) to obtain monolithic catalyst after activating, disclosed in U.S. Pat. Nos. 4,631,268 and U.S. Pat. No. 4,657,880; and (3) coating of an active washcoat on chemical inert material (such as cordierite ceramic or metal carrier) to obtain monolithic catalyst after loading active component and activating. In addition, prepared catalyst material also can be directly coated on a monolithic carrier to obtain monolithic catalyst after activating [Jan Ka{hacek over (s)}par, Paolo Fornasiero, Neal Hickey, Catal. Today, 77 (2003): 419˜449]. The advantage of the first method for preparing monolithic catalyst is that: the inner wall of structure is composed of catalyst, and the content of catalyst is obviously greater than that of catalyst prepared by the other two methods. However, the disadvantage thereof is obvious: it needs different specific extrusion equipments according to the different catalyst systems; the mechanical strength of monolithic catalyst is dependent on the natural property of catalyst system, so as to uncertainly satisfy actual application needs; a portion of catalyst is embedded in the inner wall, so as to lower the entire utilization ratio. Accordingly, in actual applications, people mostly select the other two methods to prepare monolithic catalyst, especially the third method.
Generally, monolithic catalyst prepared by the third method includes carrier, active washcoat and active component, wherein traditional carriers includes heatproof ceramic material (such as cordierite ceramic carrier), alloy material (such as Fe—Cr—Al alloy), and etc. Because the specific surface area of the carrier is relatively small (for ex. the specific surface area of cordierite ceramic carrier is smaller than 1 m2/g), the active component of catalyst can not be highly dispersed. Therefore, the active washcoat with large specific surface area must be coated. On one hand, the active washcoat can provide support for dispersion of the active component. On the other hand, the washcoat can modify active phase structure of the active component of the catalyst, in order to increase the activity and the stability of the catalyst. As a result, the method for preparing the washcoat is important to monolithic catalyst.
Presently, the method for preparing the washcoat mainly includes steps of: (1) pretreating the monolithic carrier, such as drying, calcining, and etc.; (2) preparing different coating slurry/solution according needs of catalyst system, such as slurry formed by ball-milling coating material (including boehmite and active carrier), water, adhesive, and etc.; (3) immersing the treated carrier in the prepared slurry/solution; (4) removing the excess slurry/solution in the, and then blowing out the remaining solution in channels by compressed air; and (5) drying, and calcining. In the method for preparing the washcoat, except for the properties of the coating slurry, the coating method of the coating slurry also affects the uniformity and the stability of the washcoat.
In addition, when executing the large-scale production of monolithic catalyst, the step of blowing the coating slurry by compressed air after coating easily lowers the uniformity and the stability of the washcoat.